The present invention relates to the art of poly(vinylidene chloride) copolymers and formulations for the extrusion thereof.
Copolymers of vinylidene chloride and vinyl chloride or methyl acrylate are known to be useful as barrier polymers. Extruded and coextruded shaped articles, such as films, containing a barrier layer of copolymer having from 80 to 98 percent vinylidene chloride provide excellent barrier with respect to transportation of oxygen, water, carbon dioxide and flavoring for food, medical and other high barrier packaging. Vinylidene chloride copolymers and their uses are described in numerous references, such as R. A. Wessling, Polyvinylidene Chloride (Gordon & Breach Sci. Pub. 1977) and 23 Ency. Chem. Tech., Vinylidene Chloride and Poly(Vinylidene Chloride), 764 (J. Wiley & Sons 1983), which are incorporated herein by reference.
Vinylidene chloride copolymers are susceptible to thermal degradation during extrusion. That degradation causes specks of carbonaceous material to appear in the extruded product. The level of carbonaceous material ordinarily increases at higher extrusion rates, which produce higher temperatures in the polymer. Carbonaceous material is unsightly and may cause the customer of the extruded product to reject the product. A variety of additives have been added to vinylidene chloride copolymers to help control thermal degradation and permit extrusion at higher rates. See, e.g., R. A. Wessling, Polyvinylidene Chloride at 174-76, and Johnson, Process for Imparting Stability to Particulate Vinylidene Chloride Polymer Resins, U.S. Pat. No. 4,418,168 (Nov. 29, 1983), which are incorporated herein by reference. For instance, some commercially available vinylidene chloride copolymer resins contain stabilizing amounts of tetrasodium pyrophosphate and epoxidized soybean oil. Resins containing those additives have been shown to provide containers having a barrier to oxygen of about 0.065 to about 0.15 Dow Units, with an average of about 0.10 Dow Units. One Dow Unit (D.U.) equals ##EQU1##
Vinylidene chloride copolymer compositions having higher barrier properties and equal or better extrudability would be more economical than current resins, because equivalent barrier could be obtained in extruded or coextruded products using a lesser amount of barrier resin. What are needed are extrudable and coextrudable vinylidene chloride copolymer compositions which provide improved barrier properties.